1. Field of Endeavor
The present invention relates to solid state laser systems, and more particularly to the operation of a solid state system that produces a high energy, high average power green or UV laser.
2. State of Technology
U.S. Pat. No. 5,523,262 for rapid thermal annealing using thermally conductive overcoat by Jim Fair and John Mehlhaff, assigned to Intevac, Inc., patented Jun. 4, 1996, provides the following description, xe2x80x9cRapid thermal processing (RTP) is known in the prior art as a way to anneal semiconductor wafers and other substrates in order to crystallize amorphous Si films and activate doped Si films. For example see, R. Kakkad, et al., xe2x80x9cCrystallized Si Films by Low-temperature Rapid Thermal Annealing of Amorphous Silicon,xe2x80x9d J. Appl. Phys. 65 (5), Mar. 1, 1989. Rapid thermal annealing is a process of heating semiconductor devices quickly, where the anneal time is on the order of 5 seconds. Very rapid thermal processing (VRTP) is a process where the substrate surface is heated to 1000xc2x0 C. in less than 0.5 seconds. VRTP is also known in the art. For example see, xe2x80x9cRapid Thermal Processing: How Well Is It Doing and Where Is It Going?,xe2x80x9d Mat. Res. Soc. Symp. Proc. 92, 3 (1987). One use of RTP and VRTP is solid phase recrystallization (SPC). Prior to SPC, a substrate will have on it one or more overlapping deposited silicon films which do not have a defined crystalline structure. Such silicon films are called amorphous silicon (a-Si). Heating an a-Si film to a sufficiently high temperature transforms it into a crystallized, or polysilicon, film. Another use of RTP and VRTP is to integrate doped impurities into the crystal structure of a doped polysilicon film. The heat of RTP activates the impurities in the film, and increases the conductivity of the film. In a typical substrate processing operation, several films containing patterns of circuit elements are grown on or etched from the substrate, and then selected areas of the patterned films on the substrate are doped with impurities. The substrate is then heated and cooled, thus activating the doped regions. Both a-Si films and polysilicon films can be heated effectively by exposing them to radiated light energy from a xenon arc lamp, however a-Si films generally absorb more energy in the range emitted by xenon lamps than do polysilicon films. The ability of these thin (250-2500 angstrom) films to absorb radiated energy is dependent on the thickness of the film, the amount of crystalline structure in the film, and the impurity content of the film. In general greater absorption, and therefore quicker heating, occurs in thicker films, which are more opaque to the radiated energy. The temperature reached in an exposed film is not only a function of the absorbed energy, but also a function of the rate of heat loss by conduction to the structure underlying the film. This underlying structure is made up of the substrate and any previously processed layers. The term xe2x80x9clayerxe2x80x9d is used herein interchangeably with the term xe2x80x9cfilmxe2x80x9d, however xe2x80x9clayersxe2x80x9d better describes a substrate where many films are laid one on top of the other. The rate of heat loss from a film is a function of the temperature difference between the film and its underlying structure, the heat capacity of the film, and the geometry of the boundary between the film and the underlying structure. As an example of the effect of geometry on heat loss, in a film containing an etched pattern of circuit elements, smaller features of the pattern will dissipate proportionally more heat to a cold substrate than a larger feature in a film of the same thickness. This is because a feature can dissipate heat into parts of the structure lying beyond the edges of the feature as well as into the structure directly under the feature, and a smaller structure has a larger edge-to-area ratio, giving it a higher heat transfer coupling to the underlying structure. Heat conductivity is also a function of the heat capacity of the underlying structure. Thus, if a film to be annealed is overlying a film which acts as a thermal insulator, the film being annealed will cool slower than if the film is in direct contact with a thermally conductive substrate. These unavoidable variations in radiant energy absorption and heat dissipation lead to a common problem associated with annealing a patterned substrate, namely uneven heating of an uneven film surface. Larger features, as compared with smaller features, will absorb proportionally more radiated energy, since the cross section exposed to the xenon lamp is greater, and larger features conduct heat less efficiently to the surface of the substrate. Thus, larger features tend to overheat as smaller features are heated to annealing temperatures. The overheating problem also arises when trying to uniformly heat thick, multilayer features. A thin feature is exposed to as much radiated energy as a thick feature of the same area, consequently a thick feature will heat faster due to increased opacity and less thermal coupling per unit mass to the underlying structure. Another disadvantage of radiant heating, in addition to the inability to create uniform temperatures, is that a film to be annealed must be uppermost on the substrate, where it is able to absorb the radiant energy. To meet this requirement, many annealing steps must occur in the production process, each before the layer to be annealed is covered by other layers. For example, in a typical thin film transistor (TFT) process, SPC would occur in the early stages of the process, when the layer to be recrystallized is uppermost, and implant activation would occur in the later stages of the process, when the implanted layer is uppermost. Radiant heating of thin film structures on glass substrates presents additional difficulties. Typical substrate glasses cannot withstand extended exposure to temperatures above the glass strain point (usually in the range of 575xc2x0 C. to 650xc2x0 C.). Because crystallization of a-Si films and implant activation by RTP or VRTP may require heat treatment above the glass strain point, damage to the glass substrate may occur because exposure times are too long at the power density levels typically available to RTP/VRTP (10-5000 W/cm2). To solve the problem of uneven temperatures, the substrate can be annealed in a convection furnace. However, this method also has its drawbacks. Furnace heating takes longer, and as 8 inch wafers become more common over 4 inch wafers, end-wafer heating problems will become more problematic. End-substrate heating problems also arise where the substrate is a glass substrate for a flat panel display, which can measure 8 inches across for a single device. Also, since the entire furnace chamber must be heated to the annealing temperature, the walls of the chamber are more likely to give off contaminants. The substrate can be heated using a pinpoint laser scanning in two dimensions and adjusting the intensity of the beam to compensate for the variations in energy absorption and heat dissipation. However, for this approach to work properly, the intensity control of the laser must be closely aligned with the features on the substrate, which becomes increasingly complicated as feature size decreases. Laser annealing is also undesirable because of the high power densities required and the tendency for the large temperature gradients caused by spot scanning to damage substrates.xe2x80x9d
U.S. Pat. No. 4,346,314 for a high power efficient frequency conversion of coherent radiation with nonlinear optical elements by Robert S. Craxton, assigned to the University of Rochester, patented Aug. 24, 1982, provides the following description, xe2x80x9cThe invention is especially suitable in tripling the frequency of high power laser beams having large apertures, such as the approximately 1.06 micrometer output from a neodymium glass (Nd:glass) laser which may result in increased absorption and increased neutron production by a material containing fusion fuel when irradiated by the tripled high power beam at approximately 0.35 micrometer. Nonlinear optical elements such as birefringent crystals have been used for converting the frequency of laser beams. The interaction of the beams in such elements have been studied and the principles of such interactions described at length in a paper entitled, Interactions between Light Waves in a Nonlinear Dielectric, by J. A. Armstrong, N. Bloembergen, J. Ducuing and P. S. Pershan, Physical Review, Volume 127, Number 6, (1918-1939), 1962. The authors of this paper noted certain energy transfer relationships in the nonlinear elements, which result in reconversion of the harmonic frequency components back into fundamental frequency components in the elements. This is a function of the relative optical energy (numbers of photons) of the harmonic and fundamental components in the nonlinear element. The energy in the optical element depends upon the intensity of the input laser beams passing through the element. Such beams in practice have nonconstant temporal shapes and sometimes non-uniform spatial profiles. Even when the nonlinear elements are arranged for optimum phase matching the energy relationships and realistic laser beams give rise to reconversion and loss of efficiency.xe2x80x9d
U.S. Pat. No. 5,239,408 for a high power, high beam quality regenerative amplifier by Lloyd A. Hackel and Clifford B. Dane, patented Aug. 24, 1993 provides the following description, xe2x80x9cA regenerative laser amplifier system generates high peak power and high energy per pulse output beams enabling generation of X-rays used in X-ray lithography for manufacturing integrated circuits. The laser amplifier includes a ring shaped optical path with a limited number of components including a polarizer, a passive 90 degree phase rotator, a plurality of mirrors, a relay telescope, and a gain medium, the components being placed close to the image plane of the relay telescope to reduce diffraction or phase perturbations in order to limit high peak intensity spiking. In the ring, the beam makes two passes through the gain medium for each transit of the optical path to increase the amplifier gain to loss ratio. A beam input into the ring makes two passes around the ring, is diverted into an SBS phase conjugator and proceeds out of the SBS phase conjugator back through the ring in an equal but opposite direction for two passes, further reducing phase perturbations. A master oscillator inputs the beam through an isolation cell (Faraday or Pockels) which transmits the beam into the ring without polarization rotation. The isolation cell rotates polarization only in beams proceeding out of the ring to direct the beams out of the amplifier. The diffraction limited quality of the input beam is preserved in the amplifier so that a high power output beam having nearly the same diffraction limited quality is produced.xe2x80x9d
U.S. Pat. No. 5,285,310 for a high power regenerative laser amplifier by John L. Miller, Lloyd A. Hackel, Clifford B. Dane, and Luis E. Zapata provides the following description, A regenerative amplifier design capable of operating at high energy per pulse, for instance, from 20-100 Joules, at moderate repetition rates, for instance from 5-20 Hertz is provided. The laser amplifier comprises a gain medium and source of pump energy coupled with the gain medium; a Pockels cell, which rotates an incident beam in response to application of a control signal; an optical relay system defining a first relay plane near the gain medium and a second relay plane near the rotator; and a plurality of reflectors configured to define an optical path through the gain medium, optical relay and Pockels cell, such that each transit of the optical path includes at least one pass through the gain medium and only one pass through the Pockels cell. An input coupler, and an output coupler are provided, implemented by a single polarizer. A control circuit coupled to the Pockels cell generates the control signal in timed relationship with the input pulse so that the input pulse is captured by the input coupler and proceeds through at least one transit of the optical path, and then the control signal is applied to cause rotation of the pulse to a polarization reflected by the polarizer, after which the captured pulse passes through the gain medium at least once more and is reflected out of the optical path by the polarizer before passing through the rotator again to provide an amplified pulse.xe2x80x9d
U.S. Pat. No. 5,689,363 for a long-pulse-width narrow-bandwidth solid state laser by Clifford B. Dane and Lloyd A. Hackel, patented Nov. 18, 1997 provides the following description, xe2x80x9cA long pulse laser system emits 500-1000 ns quasi-rectangular pulses at 527 nm with near diffraction-limited divergence and near transform-limited bandwidth. The system consists of one or more flashlamp-pumped Nd:glass zig-zag amplifiers, a very low threshold stimulated-Brillouin-scattering (SBS) phase conjugator system, and a free-running single frequency Nd:YLF master oscillator. Completely passive polarization switching provides eight amplifier gain passes. Multiple frequency output can be generated by using SBS cells having different pressures of a gaseous SBS medium or different SBS materials. This long pulse, low divergence, narrow-bandwidth, multi-frequency output laser system is ideally suited for use as an illuminator for long range speckle imaging applications. Because of its high average power and high beam quality, this system has application in any process that would benefit from a long pulse format, including material processing and medical applications.
Features and advantages of the present invention will become apparent from the following description. Applicants are providing this description, which includes drawings and examples of specific embodiments, to give a broad representation of the invention. Various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description and by practice of the invention. The scope of the invention is not intended to be limited to the particular forms disclosed and the invention covers all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims.
One embodiment of the present invention provides a laser system for producing a green (green=527 nm) or UV output beam for illuminating a large area with relatively high beam fluence. In this embodiment a Nd:glass laser produces a near-infrared output by means of an oscillator that generates a high quality but low power output and then multi-pass through and amplification in a zig-zag slab amplifier and wavefront correction in a phase conjugator at the midway point of the multi-pass amplification. The green or UV output is generated by means of conversion crystals that follow final propagation through the zig-zag slab amplifier.
In another embodiment the conversion crystals are configured in aspect ratio to match the zig-zag slab amplifier for producing a green or UV output beam for illuminating a large area with relatively high beam fluence. They can also be configured in a different aspect ratio than the slab with an optical element configured to change the beam shape between the amplifier and the conversion crystals.
Another embodiment provides a laser method for producing a green or UV output beam for illuminating a large area with relatively high beam fluence. It includes the steps of providing a near-infrared output, amplifying the near-infrared output, and using conversion crystals configured in an alternating z arrangement for producing a green or UV output beam for illuminating a large area with relatively high beam fluence.
An embodiment of the present invention provides a green or UV, high single pulse energy, high average power solid-state laser system. Another embodiment of the present invention provides a green or UV output by incorporating nonlinear crystals internally within the phase conjugated regenerative laser amplifier to produce the green or UV output with good near field uniformity and near diffraction limited far field beam quality.
Another embodiment of the present invention provides high conversion efficiency into the green or Uv by incorporating both control of polarization and color separation. Another embodiment of the present invention provides a green or UV output with pulse length selected from 5 nsec to as long as 1 microsecond.
Another embodiment of the present invention provides a green (green=527 nm) or UV output at either 351 nm or 263 nm or a visible output at 527 nm.
Another embodiment of the present invention provides high conversion efficiency into the green or UV by controlling the shape of the frequency converter crystals.
Another embodiment of the present invention provides high conversion efficiency into the green or UV by incorporating two crystals in an alternating arrangement of the z-axis of the frequency converter crystals to reduce the angular sensitivity by a factor of two. Another embodiment of the present invention provides high conversion efficiency into the green or UV by controlling the removal of heat deposited in the frequency converter crystals from absorbed converted or unconverted light.
Another embodiment of the present invention provides an all solid state green or UV laser system that delivers both high energy pulses and high rep rate pulses with a pulse width in the 5 nsec to 1 xcexc sec range, green or UV energy in the range of 10 to 100 J and the pulse repetition rate in the range of 5 to 10 Hz. The green or UV laser is comprised of one or more flashlamp-pumped zig-zag slab Nd:glass amplifiers, an SBS phase conjugator, a free running single frequency oscillator, and a frequency doubler or tripler or quadrupler. This green or UV laser system has the ability to illuminate a large area with high beam fluence. It is important in applications including the thermal treatment of semiconductors, thermal annealing of flat panel displays, and green or UV conditioning of optics.
In another embodiment, the flashlamp pumping of the laser is replaced with diode pumping for more efficiency and less thermal loading.
Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description and by practice of the invention.